1. Field of the Invention
The present invention relates to the pulverization of particulate matter and especially to enhancing the efficiency of bowl mills which are employed in the grinding of coarse solid material such as coal. More specifically, this invention is directed to a method for determining the optimum profile of the active surface of the grinding bowl of a rolling mill and to pulverizers having a bowl surface profile determined in accordance with such method. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly well suited for use in connection with the pulverization of coal. Pulverized coal entrained in a stream of carrier gas is widely employed as fuel in the burners of steam generators. An example of a prior art bowl mill may be seen at pages 11-24 and 16--16 of the text "COMBUSTION ENGINEERING" by Glenn R. Fryling, 1st Edition, published by Combustion Engineering, Inc. in 1966.
In the operation of a bowl mill, the coal to be ground is delivered to the center of a revolving bowl. Centrifugal forces resulting from bowl rotation cause the coal to move outwardly to the face of a grinding ring portion of the bowl where it is crushed by rotating rolls. The rotating rolls are biased toward the grinding ring either by means of hydraulic actuators or by adjustable pressure springs. The cooperation between the rotating rolls and revolving bowl reduces the size of the coal particles. In the typical bowl mill, a single pass of coal between the rolls and grinding ring produces partial pulverization, the degree of pulverization depending upon the conditions of the grinding elements and the characteristics of the particular coal which is being ground. The pulverized coal is thrown from the bowl rim into an annular hot air passage which surrounds the bowl. The pulverized coal is thus entrained in an air stream and delivered to a "classifier". The classifier will cause particles which have passed through the grinding zone but are too large for use in the firing of the steam generator to be returned to the bowl for regrinding.
The desired mode of operation of a bowl mill is to have the coal or other feed stock, i.e., the product, move to the outer edge of the grinding bowl at an adjusted speed. In prior art bowl mills, assuming that the weight of the product, the inclination of the wall of the grinding bowl relative to its axis of rotation and the direction and force of the air currents which are in part responsible for conveying the reduced product are all constants, the movement of the product will be dependent upon the coefficient of friction between the product and the bowl. Restated, it will be the coefficient of friction which determines whether the pulverized coal moves to the outside of the bowl at an adjusted speed as desired, or whether the product either "shoots through" the grinding zone or is pressed back into the middle of the bowl. For a further discussion of the mode of operation of bowl mills, reference may be had to the periodical "Aufbereitungs--Technik", No. 8, 1975, pages 401-408.
The grinding performance of a bowl mill, i.e., mill efficiency, decreases when the product "shoots through" the grinding zone. This decrease in performance results from the fact that, as briefly noted above, coarse-grained product will be separated and returned the grinding bowl where it must again pass between the rollers and grinding ring face. It should also be noted that when a significant quantity of the product "shoots through" the grinding zone, the height of the layer of product between the bowl and rollers is not sufficient to guarantee quiet running of the mill, i.e., there is no "cushion", and mill oscillations may occur. In the operation of a bowl mill, oscillations must be avoided in the interest of safety and to prolong mill service life while excessive operating noise is a matter of constant concern.